Even though I’m not a diesel mechanic doesn’t mean I cannot fix diesels. I do have a very good understanding of how they work and actually worked for a diesel repair shop for a while. I just got out of it because I don’t like all the greasy grimy heavy lifting work involved with diesel repair. Now a VW TDI is a different story. These care actually easy cars to work on but this particular problem had me stumped for a while.
When this vehicle came in it had no power and sometimes would barley run. I went through all the common things that would cause these issues. When it comes to diesels there are two main things that these vehicles need (air and fuel). Most of the time you have an engine performance with a diesel it will be related to one or both of those two things. First I went through was the easy tests such as scoping the Mass Air Flow sensor and checking TPS voltage and whatever else in the electronic engine controls that could be related to these symptoms. Now that I have eliminated these possibilities it was time to move on to the not so easy test. Hooking up inline fuel pressure tester for both the low and high pressures. You will usually find some type of mechanical failure when it comes to diesel drivability problems. So all these test have had good results. Now what? Well, if this was a gasoline engine this would have been a lot easier, but it’s not. The reason I’m saying this is because a diesel engine does not have manifold vacuum. Back to the two main things air and fuel, huh?
A diesel engine does not have a limit to the amount of air that the engine gets because there is no throttle plate. This is why they do not have engine vacuum. The engine rpm’s are directly controlled by the fuel delivery and we have already tested this. If this were a gasoline engine we could simply test the manifold vacuum to see if we are losing air volume. On a gasoline engine I will use this test for diagnosing a plugged catalytic converter. I do have an exhaust system back pressure tester so I performed that test on the VW TDI and everything looks good. Just so you know the reason for the exhaust system back pressure is sometimes when diagnosing a plugged exhaust system you can verify the problem with a vacuum gauge, but what if there are two converters and they’re on the left and right banks? What if it’s the muffler that’s causing the restriction and not the expensive catalytic converter? Well that would be an expensive mistake and that is the reason for the exhaust system back pressure tester.
No Airflow, No Power
So were on the right track, no air flow. There was probably a way of testing this on a diesel to prove my theory, but due to my other tests I knew there was only one other thing that could cause this so I went directly to the source and removed the intake manifold. The intake manifold was completely plugged! Crazy, I have never seen anything like this before. I had to literally chisel out the carbon and soak the manifold in carburetor cleaner to get out the carbon. After the manifold was cleaned I installed it and everything was back to normal.